Stabilized erythrocytes and methods therefor

ABSTRACT

A method for obtaining an improved, stabilized aldehyde-treated erythrocyte preparation having an increased hemagglutination titer. The erythrocytes are stabilized by sequential exposure to dilute solutions of pyruvic aldehyde and formaldehyde, subjected to a freeze-thaw cycle, and stored at a temperature of -20*C. or below.

United States Patent Hirata Feb. 6, 1973 54] STABILIZED ERYTHROCYTES AND OTHER PUBLICATIONS METHODS THEREFOR Ling, Brit. J. Haemat. Vol. 7, I961, pp. 299-302 [75] Inventor: Arthur Atsunobu Hirata, Wau- Kabat, Exptl. Immunochem, CC Thomas. Springfield,

kegan, Ill. 11]., 2nd Ed. 1961, pp. 542-550 [73] Assignee: Abbott Laboratories, North gg ggg ggg (1961) 27495; VOL 65 (1966) Ingraham, PSEBM, Vol. 99, Nov. 1958, 452456 [22] Filed: Dec. 24, 1970 Primary Examiner-Albert T. Meyers [2]] App! l01442 Assistant Examiner-A. P. Fagelson Related us. Application Data Ammekmbe" [63] Continuation-impart of Ser. No. 760,052, Sept. 16, [57] ABSTRACT I968, abandoned, which is a continuation-in-part of SeLNo682550NOV.13,1967abandoned A method for obtaining an improved, stabilized aldehyde-treated erythrocyte preparation having an in- [52] Us. CL 424/3 424/11 424/12 creased hemagglutination titer. The erythrocytes are 424/l3 252/408 stabilized by sequential exposure to dilute solutions of [51] lntclmmAoln U00 G01 31/00 6 33/16 pyruvic aldehyde and formaldehyde, sub ected to a [58] Field of Search (124/3 12 252/408 freeze-thaw cycle, and stored at a temperature of -20C. or below. [56] References Cited 9 Claims, No Drawings UNITED STATES PATENTS 3,057,775 10/1962 Rendon ..424/3 UR STABILIZED ERYTIIROCYTES AND METHODS THEREFOR CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending [1.8. Ser. No. 760,052, filed Sept. 16, 1968 now abandoned, which is a continuation-in-part of Ser. No. 682,550, filed Nov. 13,1967, now abandoned.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a test preparation in which erythrocytes for direct hemagglutination testing are provided which have increased activity and greatly improved storage stability. It further relates to a test preparation in which coated erythrocytes for indirect or passive hemagglutination testing are provided with enhanced activity and very superior storage stability. It further provides a method for treating the erythrocytes to impart thereto these beneficial properties.

Native erythrocytes and those treated by prior art procedures useful for direct hemagglutination testing such as blood typing suffer from the disadvantage that they deteriorate rapidly on storage so that for many purposes they are useless after about 21 days. Moreover, native cells are not susceptible to freezestorage except in special media, nor are they satisfactory after being lyophilized and reconstituted. Native erythrocytes also are sensitive to the ionic composition, pH and osmotic pressure of the suspending media, so that there exist undesirable restrictions on the scope of the tests which may be performed with them.

Erythrocytes which are coated with known antigens are a recognized preparation for use as a testing means for antibodies specific to such antigens. When an antiserum or a serum containing the suspected antibody is brought into contact with the coated erythrocytes, agglutination or clumping results which is an index that the suspected antibody is actually present in the serum. It is known in the art that such agglutination provides semi-quantitative information as to the presence and occurrence of specific antibodies and also useful information as to conditions which give rise to the presence of such antibodies.

Many problems have been encountered in preparation, storage and use of such coated erythrocytes. For example, to overcome coating problems during preparation, the art has employed coupling agents to apply antigens to the erythrocytes. The problems may also relate generally to the stability of the erythrocytes under storage conditions to their sensitivity in selected passive hemagglutination tests and to the reproducability of such hemagglutination testing. Moreover, the individual erythrocyte cells may clump or exhibit nonspecific hemagglutination, an activity which interferes with hemagglutination testing for a specific factor. Coated erythrocytes are ordinarily stored in a frozen condition, but when frozen preparations are thawed, clumping may occur or the cells may be destroyed by hemolysis.

It is desirable that erythrocytes be treated so that a variety of antigens may be coated thereon, and that such coated erythrocytes may be usefully employed as a sensitive reagent for passive hamagglutination, the testing of which is reliable and reproductable. It is also desirable to obtain antigen adsorption without requiring the presence of coupling agents or other involved steps.

It is accordingly one object of this invention to provide an erythrocyte preparation which has been treated so that it can be stored for extended periods without resulting in any hemolysis or clumping of the cells. It is a further object to provide an erythrocyte preparation useful for direct hemagglutination testing.

A further object of this invention is the provision of a method of treating erythrocytes to impart storage stability thereto while maintaining the combining sites in active condition.

Another important object of this invention is an erythrocyte preparation of the foregoing type which can be effectively coated with various antigens, and which coated erythrocytes comprise a preparation which can be effectively packaged and stored for extended periods until subjected to particular passive hemagglutination testing.

Still another important object of this invention is to provide a method for preparing stable and improved erythrocyte preparations of the type described; and for potentiating such preparations. The term potentiation refers to the conditioning of cells so as to increase the hemagglutination titer of the preparation.

In accordance with the foregoing objects as well as still other objects which will occur to practitioners, the invention is now described in detail in the following disclosure.

The following flow diagram may be referred to for a summary of the features encountered in the practice of the invention.

Contact Native Erythrocytes with Pyruvlc Aldehyde Contact Pyruvlc Aldehyde-Treated-Erythrocytes with Formaldehyde Potentlate Stable Aldehyde-Treated Erythrocytes Direct Hemagglutination Tests Coat Stable Aldehyde- Coat Stable Aldehyde- Treatcd Erythrocyte Treated Erythrocyte Preparation with an Preparation with an Antibody Antigen Potentiate Stable and Coated Erythrocytes Package and Store Stable and Coated Preparations by Freezing or by Lyoplillization r glutination Tests having substantially neutral pH to prevent hemolysis of the erythrocytes and to remove undesirable materials which may be present on the erythrocytes from the serum.

A suspension of the washed erythrocytes is prepared in the neutral buffering solution in a minor concentration of preferably less than about v/v. The erythrocytes are treated with a minor amount, preferably from 1.5 to 5.0 percent by volume of pyruvic aldehyde, relative to the volume of erythrocytes, for a time sufficient to impart desirability to the erythrocytes, preferably from 12 to 24 hours with mixing. Following the pyruvic aldehyde treatment step, the erythrocytes are washed several times to remove excess pyruvic aldehyde and any other materials which may have originated from the serum. The pyruvic aldehyde-treated erythrocytes are then exposed to a minor amount, preferably from 1.5 to 5.0 percent by volume, of formaldehyde relative to the volume of erythrocytes, for from about 12 to about 24 hours and the excess formaldehyde is removed by a plurality of washings with a buffered solution.

The foregoing pyruvic aldehyde and formaldehyde treatments may be collectively referred to as a double aldehyde treatment to attain a stabilized erythrocyte preparation. It is is an important and preferred feature of this invention that the pyruvic aldehyde treatment precede the formaldehyde treatment because it has been found that with many erythrocytes an initial formaldehyde treatment will tend to damage such cells; the preferred sequence is, therefore, recommended as the usual practice. It is understood, however, that with some erythrocytes, an initial formaldehyde treatment may not be particularly contraindicated and, therefore, in some practices an initial formaldehyde treatment might still lead to advantageous results if such a sequence is preferred.

Stabilized erythrocyte preparations may be stored for substantial periods without undesirable hemolysis, clumping, or other occurrence of objectionable features.

Human erythrocytes can be stabilized by pyruvic aldehyde-formaldehyde treatment without apparent alteration in their capacity to react with blood typing antisera. Stabilized human type A, B, or AB cells were agglutinated specifically by anti-A or anti-B serum. The sensitivity of the agglutination reaction using the stabilized cells was equal to or several fold higher than that using the native cells. Stabilized type 0 cells did not react with anti-A or anti-B serum. The above reactivities were retained for several months when the cells were stored at 4C or in freeze-storage. These stabilized cell preparations are useful as reference cells for blood typing.

The foregoing stable erythrocyte preparation may be converted to a coated stable erythrocyte preparation by contacting a sample of the aldehyde-treated erythrocytes with an antigen, followed by one or more washing steps to remove access antigen. In general, the stabilized erythrocytes are coated with antigens in a solution buffered to a pH below 7 and desirably buffered to a pH from about 3.6 to about 6. For polysaccharide antigens such as E. coli endotoxin, coating can be performed at pH 7. Superior sensitivity results when the treated erythrocytes are conditioned by agitating them different antigens, but the determine in the buffered solution for about one hour prior to coating with antigen. The coating step is usually completed in about 1 hour, but some coatings may require as long as 24 hours. A coating temperature of 24C is preferred; temperatures of 50C and higher produce erythrocyte preparations of reduced sensitivity.

These foregoing conditions may vary somewhat for practitioner may readily the preferred coating conditions by reference to agglutination occurrence with serial dilutions of a selected antiserum. For example, a rabbit may be injected with various amounts of antigen which is identical to the antigen adsorbed on the erythrocyte preparation. A serum sample would be collected in .which the antibodies would be present, and serial dilutions of this serum could be placed in V-bottom plastic trays to which an equal volume of the stable aldehydetreated erythrocyte preparation would be added. A reaction mixture containing cells and diluent serves as a control in which the cells form a button at the bottom, a negative reaction. The appearance of a matted surface would be a positive reaction of agglutination which is indicative of satisfactory coating. Non-specific agglutination must be carefully distinguished.

When reference is made herein to antigen" coating, such term should be construed as denoting a material which may be an antibody or an antigen unless particular reference is made to specific antigens or antibodies.

Representative antigens which may be successfully used to coat the stabilized erythrocyte preparations include protein antigens such as bovine serum albumin or BSA, insolubilized BSA, human serum albumin, rabbit 'y-globulin, human 'y-globulin, streptococcal M protein, ragweed antigen E, polysaccharide antigens such as E. coli endotoxin (ET), acetylated ET, succinylated ET, de-esterified ET, de-esterified and bromacetylated ET, synthetic polyglucose, miscellaneous antigens such as RNA-bacteriophage (Ribonucleic acid), QB, Entamoeba histolytica antigen and still others. It is a particular advantage of this invention that the stabilized erythrocyte preparation is suitable for coating with such a variety of substances with diverse chemical compositions.

In one embodiment of the potentiating step of this invention, the hemagglutination titer of either the stabilized or the coated erythrocyte preparation may be increased by reducing its temperature to about lC and then maintaining it at a temperature several degrees below freezing, preferably at a uniformly maintained temperature and at about 20C, for a period of about 16 weeks. Alternatively, an equally effective potentiation results within about 6 weeks by periodically raising the temperature of the stabilized or coated erythrocytes to 24C for several hours and without agitation preferably at intervals of about six or seven days. Cycles having a length of several hours to a few days were ineffective.

In another embodiment of the potentiating step, the erythrocytes were treated with a dilute solution of oxidizing agent, sodium periodate being a suitable reagent.

In still another embodiment of this invention the stabilized erythrocyte preparation may be potentiated by coating the cells thereof with fibrinogen prior to coating with the antigen. Bovine fibrinogen is a suitable potentiating agent.

The potentiated stabilized or coated erythrocyte preparation may be stored in a lyophilized state or at a temperature of about -190C until needed for hemagglutination tests. The manner of performing these tests is well understood by those skilled in the art.

In general, an animal serum sample with the suspected antibody is combined with the coated erythrocyte in a liquid vehicle to determine if any agglutination occurs. Likewise, the stabilized erythrocyte preparations may be coated with a specific antibody so that in subsequent hemagglutination testing, the erythrocytes will agglutinate in the presence of a free specific antigen. Other varieties and modifications in the hemagglutination testing process will occur to practitioners and all such tests may be practiced to ad vantage with the improved stable and coated erythrocyte preparations.

The passive hemagglutination test may be used to detect the presence of y-globulin on the cells as in Coombs test, detection of bacterial infection as in the presence of endotoxin antigen, detection of viral infection, the typing for histocompatibility, the detection of autoimmune diseases and other phenomena of this type. The coated and stable erythrocyte preparations may also be used as a testing reagent in chemical analyses to detect presence of antigen in a quantitative or semi-quantitative fashion.

The following examples are presented to teach various embodiments for practicing the invention, but while such embodiments include those best contemplated at present for practicing the invention, it should be understood that they do not represent exclusive embodiments. Neither do they necessarily include what may prove to be preferred forms in future practice.

EXAMPLE 1 Stable Aldehyde Treated Erythrocyte Preparation A 20 ml. sample of whole blood is removed from a 2-3 kg. rabbit by cardiac puncture, and said sample is collected in a syringe containing an equal volume of sterile Alsevers solution. This solution is generally isotonic and contains a sodium citrate anticoagulant, sodium chloride, dextrose and distilled water. The withdrawn sample is allowed to stand overnight at about 4C, and it is then centrifuged at 1500 rpm. for minutes to obtain about 10 ml. of packed rabbit erythrocytes. The packed erythrocytes are then combined with 10 volumes of a 0.11M phosphate buffer solution, pH 7.2. This buffered solution is prepared from monopotassium phosphate and disodium phosphate with vortex mixing. The mixture of erythrocyte and buffer solution is centrifuged and then subjected to five separate washings with 10 volumes of the foregoing buffered solution to remove serum protein and any other serum materials. The washed erythrocytes are then resuspended in the foregoing buffered solution to attain an 8% v/v rabbit erythrocyte suspen- SIOI'I.

To a 500 ml. Erlenmeyer flask is added 125 ml. of the phosphate buffered solution containing 3 percent pyruvic aldehyde v/v and 125 ml. of the foregoing 8 percent erythrocyte suspension, and the mixture is stirred about 18 hours at room temperature. The mixture is then filtered through a gauze pad to remove any large debris and the erythrocytes are then washed on five separate occassions with 10 times their volume of the phosphate buffered solution. The pyruvic aldehyde treated erythrocyte preparation is then resuspended in the foregoing buffered solution in a 10 percent concentration v/v.

A ml. volume of the foregoing pyruvic aldehyde treated suspension is diluted to 8 percent and mixed with 125 ml. of the phosphate buffered solution containing 3 percent formaldehyde v/v. The mixture is agitated overnight with a magnetic stirrer at room temperature, and the mixture is then filtered through a gauze pad to remove any possible debris. The pyruvic aldehyde and formaldehyde treated erythrocytes are then washed on five different occassions with 10 times their volume of the foregoing buffered solution. The aldehyde treated erythrocyte preparation is then resuspended as a 10 percent concentration in the foregoing buffered solution and transferred to glass containers and stoppered for storage. The foregoing aldehyde treated erythrocyte preparation is stable for at least 2 months at reduced temperatures of 4C, or for indefinite periods frozen in liquid nitrogen.

The stabilized erythrocyte preparation may be potentiated, but in the preferred process the potentiation step is reserved until after the erythrocytes have been coated.

The stabilized erythrocyte preparation made according to the process of this invention possesses most, if not all of the combining sites present on the native erythrocyte, combining sites referring specifically to sites which may react with antibody or virus.

EXAMPLE 2 Potentiated Stabilized Erythrocyte Preparation This example will illustrate the increase in hemagglutination titer achieved by attaching fibrinogen to the cells of the stabilized erythrocyte preparation. Bovine fibrinogen at a concentration of l microgram per milliliter in 0.1 M acetate'buffered solution, pH 4.0, was heated at 60C for 1 hour and cooled to room temperature. A 10 percent suspension of the double aldehyde treated erythrocytes obtained by the process of Example l were added to the fibrinogen solution and stirred for 1 hour at room temperature. The potentiated stabilized erythrocytes were washed with a phosphate buffer solution, pH 7.2, and the erythrocyte concentration adjusted to 10% v/v. These cells were later used with superior results for blood type determinations.

EXAMPLE 3 Potentiated, Stabilized, Coated, Erythrocyte Preparation One milliliter of the 10 percent suspension of potentiated stabilized erythrocyte preparation obtained by the process of Example 2 was washed with a 0.1 M acetate buffer solution, pH 4.0, and resuspended in 9 milliliters of the acetate buffer solution at 24C. One milliliter of the acetate buffer solution containing 1 milligram of BSA is added. The mixture was stirred at room temperature for 2 hours and then washed 5 times with an excess of phosphate buffered solution, pH 7.2. These cells were later used for passive hemagglutination reactions.

EXAMPLE 4 erythrocytes are not hemolized or clumped by this lowered pH level. The washed erythrocyte preparation cells are resuspended in 9 ml. of the acetate buffered solution at 24C, pH 4.0 and agitated at this pH and temperature for 1 hour; then 1 ml. of the acetate buffer solution containing 1 mg. of BSA is added. The mixture of antigen and stable aldehyde treated erythrocytes are rotated at room temperature for 2 hours and then subjected to 5 separate washings with excess volumes of a phosphate buffered solution, pH 7.2. The BSA coated stable erythrocytes are resuspended in the foregoing phosphate buffer at concentrations of 0.5% v/v, and ml. aliquots of this suspension are packaged in ml. glass bottles and stoppered. The packaged predetermined amount of the coated stable aldehyde treated erythrocyte preparation is then quick frozen in a liquid nitrogen freezer and stored at 20C.

EXAMPLE 5 Potentiation of Coated Stable Erythrocyte Preparation Treated erythrocytes, coated as in Example 4, but at a pH of 3.6, having been frozen at l 96C and stored at 20C for 6 days, are thawed to room temperature of 24C without agitation, maintained at 24C for 2 hours, and after refreezing at 196C, are returned to 20C. The process is repeated once a week for 6 weeks.

The potentiated coated erythrocytes are then frozen at 196C and stored at that temperature until needed for the hemagglutination test. Alternatively, the potentiated coated cell suspension can be lyophilized and reconstituted by adding water before use.

This process increases the titer -fold while only doubling the non-specific hemagglutination titer, a practically insignificant increase. Further repeated freeze-thaw cycles results in a further increase in hemagglutination titer but also produces a concurrent significant increase in the non-specific hemagglutination titer, so that little advantage is realized from the extended processing.

The optimum number of freeze-thaw cycles is related to a specific coated cell preparation but the optimum will be recognized by those skilled in the art as occurring at that cycle beyond which the increase in hemagglutination titer is accompanied by an appreciable increase in the non-specific hemagglutination titer. A coated cell preparation which was optimal for the standard serum was found to be optimal also for other antisera. Thus, successive standard sera can be standardized against the original selected anti-serum.

EXAMPLE 6 Sensitivity of Erythrocyte Preparation Coated with Bovine Serum Albumin Following the procedures of Examples 1 and 4, an erythrocyte preparation is obtained in which rabbit erythrocytes are coated with Bovine Serum Albumin (BSA). A related procedure is followed to coat another erythrocyte preparation with BSA, but the erythrocytes of this sample are stabilized only with pyruvic aldehyde and not with the aldehyde treatment sequence according to the disclosure of this invention. The hemagglutination titers of both preparations are compared in the following Table l to illustrate the greater sensitivity of the BSA coated erythrocyte preparation which has been stabilized with the double aldehyde treatment. Also illustrated are the results of the potentiation step.

EXAMPLE 7a Potentiated Stabilized Erythrocyte Preparation This example will illustrate still another embodiment of the potentiating step of this invention. Human erythrocytes were processed according to the procedure of Example 1 to obtain stabilized human erythrocytes suspended in a phosphate buffered solution, pH 7.2. The stabilized human erythrocytes were then treated at 24C for 15 minutes with 0.00035M sodium periodate solution and then washed free of this reagent with phosphate buffered solution pH 7.2. When subjected to direct hemagglutination reaction against rat antiserum, the stabilized cells gave a reaction at dilution of 1 to whereas the stabilized cells potentiated with the oxidation reaction exhibited hemagglutination at a dilution of l to 2,000.

EXAMPLE 7b TABLE I Comparison of Hemagglutination Reaction of Erythrocyte Preparations Condition Coating Duration pH (Hours) Hemagglutination Coated Cells Titer BSA-coated Pyruvic aldehyde treated 4 2 erythrocytes BSA-coated Pyruvic aldehyde and formaldehyde 4 2 treated stabilized erythrocytes BSA-coated Pyruvic 'aldehyde and formaldehyde treated stabilized erythrocytes after six cycles of freeze-thaw potentiation (Example 5) BSA-coated, stabilized oxidation 4 2 potentiated,erythrocytes (Example 7b) BSA coated,

fibrinogen potentiated, 4 2 stabilized erythrocytes (Example 3) 3.6 l l/l,280,000

EXAMPLE 8 Representative Variations in Conditions For Coating Aldehyde-Treated Erythrocyte Preparations with Various Antigens A number of representative antigens are selected for coating an erythrocyte preparation prepared according to the procedure set out in Example I. The coating procedure of Example 4 is generally followed.

The following data illustrates recorded observations which are readily determined by skilled practitioners by noting the occurrence of positive agglutination with serial dilutions of rabbit serum as previously described.

Amount Duration Hours Antigens (mg/10 ml.) pH (at 24 C) Bovine Serum Albumin (BSA) 10 4 2 lnsolubilized BSA 0.] 4 2 Rabbit Globulin 0.01 5 H6 Endotoxin and Derivatives 0.01 7 Bi Polyglucose 1.0 4 r5 Bacteriophage, QB 0.3 5 4 Sheep leutenizing hormone 0.004 4 4 Bovine Serum Albumin (BSA) 10 4 1 Bovine Serum Albumin (BSA) 1.0 3.6 1"

*Plus 1 hour agitation at pH 4 prior to antigen addition. "Plus 1 hour agitation at pH 3.6 prior to antigen addition.

The invention may now be practiced in the many various ways which occur to those skilled in this art, and all such modifications in practice will comprise a part of the concept behind the disclosed embodiments. The invention is now defined by the terms of the following claims which are given further meaning by the language of the preceding description.

lclaim:

l. A method for obtaining an improved, stabilized aldehyde-treated erythrocyte preparation which is useful in passive hemagglutination procedures, said method comprising the steps of: preparing a suspension of washed erythrocytes in a neutral buffered solution; treating said erythrocyte suspension with from L5 to 5.0 percent by volume of pyruvic aldehyde per volume of erythrocytes for from 12 to 24 hours at room temperature; washing the aldehyde-treated erythrocyte suspension with said buffered solution to remove the excess pyruvic aldehyde; treating the washed aldehydetreated erythrocyte suspension with from 1.5 to 5.0 percent by volume of formaldehyde per volume of erythrocytes for from 12 to 24 hours; washing the resulting stabilized erythrocyte preparation with said buffered solution to remove the excess formaldehyde; cooling said erythrocyte preparation to about lC, increasing the temperature to about 20"C, and storing the resulting stabilized erythrocyte preparation at a temperature no greater than 20C.

2. The method of claim 1 wherein said erythrocyte preparation is cooled to -l 90C. in liquid nitrogen.

3. The method of claim 1 wherein the temperature of the erythrocyte preparation is maintained at a temperature of about 20C. for about 6 days, raised to a temperature of about 24C. for 2-4 hours, recooled to about l90C. and then returned to and stored at a temperature of about -20C.

4. The method of claim 3 wherein the freeze-thaw cycle is repeated from six to nine times.

5. A method of potentratmg the stabilized erythrocytes obtained by the process of claim 1 by treating the erythrocyte preparation with a dilute solution of a suitable oxidizing agent at a temperature of from 20 to 30C. for from 10 to 20 minutes and washing the potentiated stabilized erythrocyte preparation.

6. The process of claim 5 wherein said erythrocyte preparation is treated at 24C. for 15 minutes with a 0.00035 M sodium periodate solution and then washed with a phosphate buffered solution to remove the oxidizing agent.

7. The method of claim 6 wherein said neutral buffering solution is a pH 7.0-7.2 phosphate solution.

8. The stable erythrocyte preparation obtained by the method of claim 1.

9. The stable erythrocyte preparation of claim 8 in lyophilized form. 

1. A method for obtaining an improved, stabilized aldehyde-treated erythrocyte preparation which is useful in passive hemagglutination procedures, said method comprising the steps of: preparing a suspension of washed erythrocytes in a neutral buffered solution; treating said erythrocyte suspension with from 1.5 to 5.0 percent by volume of pyruvic aldehyde per volume of erythrocytes for from 12 to 24 hours at room temperature; washing the aldehyde-treated erythrocyte suspension with said buffered solution to remove the excess pyruvic aldehyde; treating the washed aldehyde-treated erythrocyte suspension with from 1.5 to 5.0 percent by volume of formaldehyde per volume of erythrocytes for from 12 to 24 hours; washing the resulting stabilized erythrocyte preparation with said buffered solution to remove the excess formaldehyde; cooling said erythrocyte preparation to about -190*C, increasing the temperature to about -20*C, and storing the resulting stabilized erythrocyte preparation at a temperature no greater than -20*C.
 2. The method of claim 1 wherein said erythrocyte preparation is cooled to -190*C. in liquid nitrogen.
 3. The method of claim 1 wherein the temperature of the erythrocyte preparation is maintained at a temperature of about -20*C. for about 6 days, raised to a temperature of about 24*C. for 2-4 hours, recooled to about -190*C. and then returned to and stored at a temperature of about -20*C.
 4. The method of claim 3 wherein the freeze-thaw cycle is repeated from six to nine times.
 5. A method of potentiating the stabilized erythrocytes obtained by the process of claim 1 by treating the erythrocyte preparation with a dilute solution of a suitable oxidizing agent at a temperature of from 20* to 30*C. for from 10 to 20 minutes and washing the potentiated stabilized erythrocyte preparation.
 6. The process of claim 5 wherein said erythrocyte preparation is treated at 24*C. for 15 minutes with a 0.00035 M sodium periodate solution and then washed with a phosphate buffered solution to remove the oxidizing agent.
 7. The method of claim 6 wherein said neutral buffering solution is a pH 7.0-7.2 phosphate solution.
 8. The stable erythrocyte preparation obtained by the method of claim
 1. 